Changing set method defaults for arm vs intel Mac

Changing default multiprocessing start method to "fork" for ARM Macs and leaving default "forkserver" for intel macs.

Author: bchen93

pymc/parallel_sampling.py

@@ -408,7 +408,11 @@ def __init__(
             # Closes issue https://github.com/pymc-devs/pymc/issues/3849
             # Related issue https://github.com/pymc-devs/pymc/issues/5339
             if platform.system() == "Darwin":
-                mp_ctx = "fork"
+                if platform.processor() == 'arm':
+                    mp_ctx = 'fork'
+                else: 
+                    mp_ctx = "forkserver"
+
             mp_ctx = multiprocessing.get_context(mp_ctx)
 
         step_method_pickled = None